A heat-sensitive recording sheet as used herein refers to a sheet on which images can be recorded by causing physical and chemical changes of substances within the sheet by heat energy, and many processes thereof have been investigated.
A wax type heat-sensitive recording paper which relies on the physical change of a substance caused by heat energy has been described in the prior art, and is used to make electrocardiograms. Heat-sensitive recording sheets which rely on the chemical change of a substance caused by heat energy have been proposed based on various color-forming mechanisms as the chemical change. Among these, a heat-sensitive recording sheet which forms color due to reaction of two compounds is the most typical example.
A heat-sensitive recording sheet forming color due to the reaction of two compounds can be prepared by dispersing two heat-reactive compounds finely, adding a binder so that two compounds are separated from each other, and then coating these compositions on a support. The record can be obtained in this heat-sensitive sheet by heating the sheet to melt one or both of these compounds, to bring about contact of the two heat reactive compounds with each other, and thereby to initiate the color forming reaction.
The two heat-reactive compounds are generally an electron donating compound and an electron accepting compound.
The heat-sensitive recording sheet forming color due to reaction of the two reactive compounds has a number of advantages, such as, (1) color forming is primary color forming and so development is unnecessary, (2) the paper quality is nearly that of normal paper and (3) the recording sheet is easy to handle. Particularly when a colorless dye is used as an electron donating compound, the heat-sensitive recording sheet has the further advantages that (4) color density is high and (5) heat-sensitive recording papers having various color forming hue can easily be prepared. Accordingly, the above heat-sensitive recording sheet is the most widely used.
Recently, heat-sensitive recording sheets have been used in the fields of faxcimile sending, a recording apparatus, and a printing. With the increased use in the faxcimile field, recording speed has been shortened. Improvement for shortening the heat pulse toward a sheet, that is, a color forming property obtained with low heat energy, i.e., a good heat response property, has strongly been desired.
Particularly when the environmental temperature is low, more specifically, when the temperature is 10.degree. C. or less, a sufficient printed density can not be obtained in a general heat-sensitive recording sheet.
On the other hand, since the color forming of the heat-sensitive recording sheet is primary color forming, a problem arises because the electron donating colorless dye precursor and the electron accepting compound are reacted not only by heat but also by solvents and the like, such as those contained in a felt pen or a fluorescent pen which is commonly used for writing on the sheet. Because most of the materials for making the heat-sensitive recording sheet are organic compounds, which have high solubility to the solvents, the heat reactive compounds are easily brought into contact with solvents, and therefore they easily react with a solvent. In order to remove the above defect, efforts have been made to provide a protective layer having solvent resistance on the heat sensitive color forming layer. However, the manufacturing process thereof is complicated.